Parisot has been a consistent critic of the Front National's economic policies, claiming that they will damage France.
In a recently published book, Parisot characterized Le Pen's ideas and "hatred" as "dangerous". She is scathing about the party's economic policies, particularly its demand for France to leave the euro.
In an interview with weekly magazine Paris Match she said the book "has the aim of stopping Marine Le Pen getting through to the second round of voting in the presidential election".
In the French electoral system, the field of candidates is narrowed down to the two front-runners who then face each other in a final vote. In 2002, the former leader of the Front National, Le Pen's father Jean-Marie, managed to beat the Socialist candidate, Lionel Jospin, to come face-to-face with Jacques Chirac in the second round of voting.
"If there was the misfortune of this happening," wrote Parisot, "I would organize a special meeting of the executive council of Medef. I don't doubt that, just as in 2002, it would call on voters to block the Front National."
Weekly magazine L'Express has seen a letter sent by Le Pen to the 45 members of Medef's executive committee.
In it the Front National leader questions whether Medef is sticking to "the principles that Medef claims…of no interference in the democratic electoral debate and no voting guidance."
"I am writing to you directly to know if you support this declaration made in your name and if you associate yourselves with this clear declaration of war against a political party," she wrote.
Marine Le Pen claims the Medef leader's book contains "an impressive number of errors, lies and nonsense". She has also invited the Medef leader to have a public debate on the issues.